1. Field of Invention
The present invention relates to an organic electroluminescent element, a method for manufacturing the organic electroluminescent element, and a light emitting display device using the organic electroluminescent element. More specifically, the present invention relates to an organic electroluminescent element having an element structure that can reduce damage to an organic layer during electrode formation and can facilitate the injection of charges from the electrode into the organic layer, and a method for manufacturing the organic electroluminescent element and a light emitting display device using the organic luminescent element.
2. Description of Related Art
An organic electroluminescent element, which comprises a luminescent material-containing organic luminescent layer held between a pair of electrodes and emits light by applying voltage across both the electrodes (hereinafter sometimes referred to as an organic EL element), has advantages such as high visibility by virtue of self-color development, high impact resistance due to the nature of a whole solid state element unlike liquid crystal elements, a high response speed, low susceptibility to temperature change, and a large view angle, and attention has been drawn to the utilization of the organic electroluminescent element as a luminescent element in display devices.
The organic EL element has a basic construction of a laminated structure of anode/organic luminescent layer/cathode. The organic EL element generally has hitherto been of a bottom emission type that is manufactured by stacking an anode formed of a transparent electroconductive film on a transparent base material such as a glass substrate and then stacking an organic luminescent layer and a cathode in that order and takes out light from the anode side.
In recent years, attention has been drawn to a top emission-type organic EL element that, in the above construction, uses a transparent electroconductive film in the cathode and takes out light from the cathode side. In the top emission-type organic EL element, since the cathode and the anode are formed of a transparent electroconductive film, a luminescent element which is transparent as a whole can be realized and, thus, both side emission can be realized. The transparent luminescent element is advantageous in that, since any desired color can be adopted as a background color, a display device which can be in a colored state even at a time other than during light emission can be provided and, thus, excellent decoration can be realized. Further, in the top emission-type organic EL element, since emitted light is not shielded by TFT (a thin-film transistor) which is an active drive element, a display device having a high opening ratio can be realized.
In the top emission-type organic EL element, an organic luminescent layer is generally provided between the anode and the cathode. Various forms of construction of the top emission-type organic EL element are possible. In one example of the construction, the cathode comprises an electron injection layer formed of aluminum (Al) or the like and a transparent electroconductive film formed of ITO (indium tin oxide) or the like, the electron injection layer being disposed on the organic luminescent layer side.
Since, however, the transparent electroconductive film formed of ITO or the like is generally formed by sputtering, in the organic EL element having the above construction, the formation of the transparent electroconductive film on the electron injection layer poses a problem of causing lowered luminescence characteristics as a result of damage to the organic luminescent layer or the electron injection layer due to impact of sputtered particles or argon ions (Ar+), ionized electrons or the like during sputtering, for example, lowered current density, lowered luminescence efficiency, and the occurrence of leak current.
Further, the use of plasma in the formation of the transparent electroconductive film poses a problem of causing lowered luminescence characteristics as a result of undergoing the same damage as described above due to exposure of the organic luminescent layer or the electron injection layer to a plasma atmosphere.
Furthermore, in the formation of the transparent electroconductive film, problems occur including a problem of causing lowered electron injection properties as a result of oxidation of a highly reactive metal such as aluminum (Al) constituting the electron injection layer by oxygen in the atmosphere or oxygen released from a target, for example, lowered current density characteristics, lowered luminescence efficiency, and increased dark spots.
In order to solve the above problems, organic EL elements comprising various layers provided between the organic luminescent layer and the transparent electroconductive film (upper electrode) have been proposed. For example, Japanese Patent Application Laid-Open No. 77651/2003 proposes an organic EL element comprising a sputtered protective layer that is formed of gold, nickel, or aluminum and is provided between an organic luminescent layer and a transparent electroconductive film. Further, Japanese Patent Application Laid-Open No. 127740/2004 proposes an organic EL element comprising a cathode having a two-layer structure of a first cathode and a second cathode and an electron transport protective layer provided between the first cathode and the second cathode, the electron transport protective layer being formed of an electron transport organic material such as bathocuproine (BCP) doped with an alkali metal or an alkaline earth metal (for example, lithium (Li), cesium (Cs), barium (Ba), strontium (Sr), or calcium (Ca)). Further, Japanese Patent Application Laid-Open No. 223377/1998 proposes an organic EL element comprising a cathode formed of a thin metal film (a Ca film) and a protective film that covers the thin metal film and is formed of a semiconductor (for example, ZnSe) having a broad bandgap. Furthermore, Japanese Patent Application Laid-Open No. 227943/2004 proposes an organic EL element comprising a hole injection layer having a thickness of 30 nm to 1000 nm that is provided between an organic luminescent layer and a transparent electroconductive film (an upper anode). Furthermore, Japanese Patent Application Laid-Open No. 296234/2004 proposes an organic EL element comprising a buffer layer that is provided between an organic luminescent layer and a transparent electroconductive film (an upper electrode) and is formed of a phthalocyanine compound doped with a metal such as gold (Au), platinum (Pt) or silver (Ag). Furthermore, Japanese Patent Application Laid-Open No. 265792/2007 proposes an organic EL element that, in order to keep a balance between holes and electrons injected into an organic luminescent layer, comprises a laminate of semi-transparent cathode layer/transparent buffer layer/transparent electroconductive protective layer provided on the organic luminescent layer and has a contact region, provided in a non-display region, where the semi-transparent cathode layer is in contact with the transparent electroconductive protective layer, whereby current is allowed to flow through the semi-transparent cathode layer and the transparent electroconductive protective layer to efficiently supply electrons into the organic luminescent layer.
In the organic EL elements proposed in the above patent documents, various layers are inserted into between the organic luminescent layer and the transparent electroconductive film (upper electrode). In the top emission-type organic EL element, however, transparency is required of the layer to be inserted and, at the same time, electron transport properties and electron injection properties necessary for supplying charges into the organic luminescent layer should be satisfactorily high. In particular, unsatisfactory electron transport properties and electron injection properties pose a problem of causing a lowering in current-voltage characteristics due to an increase in electric resistance as the whole element and, at the same time, causing a lowering in luminescence characteristics due to loss of balance between holes and electrons injected into the organic luminescent layer.
Further, in the organic EL element proposed in Japanese Patent Application Laid-Open No. 265792/2007, in order to allow current to flow through the semi-transparent cathode layer, the thickness of the semi-transparent cathode layer should be large. This sometimes leads to lowered transparency of the element. Furthermore, since the semi-transparent cathode layer is formed of a highly reactive material such as an alkali metal as a simple substance, in the region where the semi-transparent cathode layer is in contact with the transparent electroconductive protective layer, the transparent electroconductive protective layer sometimes undergoes damage such as oxidation in the formation of the transparent electroconductive protective layer.